A number of containers used for containing beverages and foods are metal containers made from steel sheets such as nickel-plated steel sheets, tin-plated steel sheets, or tin-based-alloy-plated steel sheets. These metal containers need to be painted before or after the can-making process. In recent years, film lamination has taken the place of painting in a number of cases in the manufacture of such metal containers to reduce waste resulting from painting materials such as waste solvents and waste gases such as carbon dioxide from the viewpoint of global environment conservation.
Steel sheets for containers used as a base for painting or film lamination are often steel sheets treated against corrosion by chromating using, for example, hexavalent chromate to ensure corrosion resistance and adhesion between a steel sheet and paint or film (see, for example, Patent Literature 1). These chromate-treated steel sheets are further provided, where necessary, with a coating layer of organic resin disposed on the chromate treatment coating.
However, because hexavalent chromium used for chromate treatment is harmful to the environment, recently there has been a trend toward replacing the chromate treatment hitherto applied to steel sheets for containers. In contrast, because the chromate coating formed by the chromate treatment on a steel sheet has an enhanced level of corrosion resistance and adhesion to paint for film), these properties are expected to significantly decrease without such chromate treatment. Therefore, there is a now a trend toward performing anti-corrosion treatment on the surface of a steel sheet for containers in lieu of chromate treatment in order to form an anti-corrosion layer having good corrosion resistance and adhesion to paint (or film). Among the anti-corrosion treatments that have been proposed to replace the chromate treatment, there have been proposed the following surface treatment methods.
Patent Literatures 2 and 3, for example, each propose a steel sheet for containers having, on at least one side of the steel sheet, a chemical conversion coating layer comprising at least two kinds of coatings selected from the group consisting of a zirconium coating containing a given amount of zirconium, a phosphoric acid coating containing a given amount of phosphoric acid, and a phenolic resin coating containing a given amount of phenolic resin, wherein any given particle in the chemical conversion coating layer has a size not larger than a given value.